vt(4): Pause the vt_flush() timer when the screen is up-to-date

The timer is restarted whenever a window buffer is marked as dirty or
the mouse cursor moves.

There's still room for improvement. For instance, we should not mark a
window buffer as dirty when this window isn't displayed.

Review: https://reviews.freebsd.org/D683
Reviewed by: ray@
Approved by: ray@
MFC after: 1 week